1. Field of the Invention
This invention relates to the use of high and low density brines in wellbores, and it more particularly relates to the complete removal of drilling fluid (mud, solids and oil) from the wellbore prior to introduction of the solids-free brines.
2. Description of the Prior Art
Aqueous brine solutions of high density are used in wellbores employed in the production of petroleum. These solutions have been used as both drilling, completion and packer fluids especially in deep wells subject to high formation oil and gas pressures at elevated temperatures. These brines can be formed of the sodium, potassium, calcium and zinc salts with chloride and bromide ions. These aqueous fluids may include corrosion inhibitors and other salts such as soda ash. The density of these salt type well fluids depends on the particular salt, or mixture of salts, and their concentration in the aqueous well fluid. Usually, these salt type well fluids have a density in the range of between about 8 and 19 pounds per gallon.
The salt type well fluid should be solids-free in its use as a well fluid. If there are solids in a packer or completion fluid, they can cause serious injury to a producing formation by plugging of the pore spaces therein or even of the perforations and channels provided to induce fluid flows between the formation and well bore. If there are solids in a packer fluid, the solids will precipitate with time upon the packer. As a result, these solids deposit make it difficult to disconnect the tubing from the packer with a resultant costly well workover.
The high density brine can be prepared at the wellsite by dissolving the prescribed amount of salt into the aqueous phase, which phase is principally fresh or sea water but it can include various inhibitors for preventing pitting, corrosion, etc. The mixture is circulated or agitated in the surface mud system equipment until there are no undissolved salt solids. Naturally, the problems of adding salts to be dissolved in the aqueous well fluid became progressively more severe as the density increases, both in time, manpower and equipment requirements. Importantly, the surface mud system must be scrupuleously clean of any solids, especially the solid and oil portions of drilling mud. Otherwise, the prepared brine must be filtered to remove solids until the solids content has been reduced to 0.2% or lower.
At present, vendors will deliver to the wellsite the prepared high density brine of a desired density and combination of selected ingredients. The delivery of brine usually requires several changes in containers. For example, the brine is moved from the vendor tanks to truck transport, offshore supply boat and into the rig mud system. In most circumstances, the brine becomes contaminated from the mud system by undesired solids, including residual water wetted solids and/or oil based drilling mud, weighting agents such as barite, rust, salt, silt, ferrous and ferric precipitates and sand, and other undissolved materials. Contaminating liquids such as mud bases, lubricants and diesel fuel can also be present in the mud system and entrained in the brine.
As a practical result, present day rig practices, especially offshore, require full stream filtration (usually in cartridge filters) of the brine so that solid levels of 0.2% or less are reached immediately before the brine is sent into the well bore.
A process has been developed for removing the contaminating solids from high density, salt-type (brine) aqueous drilling, completion and packer fluids before their placement into a well bore. This process is described in our pending application Ser. No. 310,653 filed Oct. 13, 1981, which application for descriptive purposes is incorporated herein. Thus, the solids contaminated brine can be cleaned by our unique process, specifically before it enters the surface mud equipment of the well system.
Although the brine can be made solids-free at the rig, it is preferred to clean the well system of drilling fluid or mud solids oil, etc. before introduction of the brine into it. One of the main problem areas in removing drilling fluid is from the wellbore equipment which includes the tubing or well pipe and the annulus between it and the casing or surrounding wellbore.
Many chemical washes have been proposed and used to remove drilling fluid from the wellbore equipment prior to introduction of the solids-free brine. For example, circulating chemical washes using water with surfactants, viscosifiers and gel agents, chemical aids such as sodium tetraphosphate, weighting agents such as barite have been used under turbulent flow conditions for achieving effective mud removal from the wellbore equipment. In some cases, the chemical washes are used as aqueous spacers for increased drilling fluid removal much in the same manner as used to displace drilling fluid prior to a cement slurry used in well cementing operations.
Usually the chemical washes were designed for turbulent or upper laminar flow with aqueous phases of fresh and salt water continuous, and for plug flow using gelled water-based phases. The gelled water phase or plug was sufficiently viscous to attempt to reduce settling of drilling fluid from it, especially solids. If oil based drilling fluid was being displaced, diesel oil and emulsifier are added to increase removal of the drilling fluid and its mud cake. Some chemical washes combined turbulent and plug flow aqueous phases to enhance the displacements of drilling fluid from the wellbore equipment, especially prior to cementing operations or introduction of completion brine. These chemical washes were used, in some operations, in a single pass travel through the wellbore equipment for removing the drilling fluid so as to save rig time.
The present process is an improved process for removing substantially all of the drilling fluid or mud, including solids such as barite, bentonite, cement, etc. and oil materials, from the well bore equipment prior to entry therein of the solids-free completion and packer brine. As a result, the brine remains substantially free of solids but if filtration is needed, the filter loading remains very small and does not involve any significant expense in rig time. In this process the wellbore equipment is cleaned by treated water at turbulent flow placed as a slug between the drilling fluid being displaced and the solids free brine being introduced into the wellbore equipment. Aqueous spacers can preceed or follow the treated water. The overall rig filtering time to practice the present process requires only two-to-three hours. High angle directional wells (e.g. 70 degrees) will achieve considerably greater savings in filtering and trip time using this process.